Server spec question

We have a remote site in which we are thinking of putting in 3cx. There will be up to 15 users, with up to 15 calls at once (though the chances are that most of the time there will be many fewer users and simultaneous calls).

We are putting in a new windows based active directory controller that will also do file duties. We have the option of using this, or using a virtual machine running on this. Alternatively we could use a lesser spec machine perhaps running Windows 7.

Does anyone have any examples of similar size set up and server details or a good rule of thumb for server sizing?

We're looking for the most cost effective way to get 3cx in there. Clearly it needs to be workable, but I think it can also be scalable.

It would be cheaper, I think, to add some upgrades to the existing server (ram etc) rather than get a dedicated box. I would also prefer to virtualise the 3cx server as it's then easier to upgrade the virtual machine if required or move it to another box for maintenance.

As for the use of the AD and file server roles, it's hard to say. The office, whilst live, isn't really used currently. When the users move in to the office it really will be just for local authentication (they work mainly on TS boxes at HQ so they don't do much locally) and to store some files for local use (again as they work on TS servers at HQ there will be very little files).

If the role of that server grows then we can look at adding/upgrading servers if required but for now we need to get the phones working on a budget.

The server is currently a 4GB Windows 2008 R2 server with 2 x 500GB sata drives in a mirrored array. The raid controller can take SAS drives if required. If we virtualise I would be keen to look at using a Windows workstation varient rather than the server version just for costs, but again this can change and be changed in the future.